This invention relates to a laser radar vision apparatus capable of producing a three-dimensional image of a distant target.
In particular, it relates to a lightweight, multiplexing, electronic apparatus for capturing a three-dimensional (3D) image of a distant target at high-spatial and high-range resolution in the atmosphere or in space with a single laser pulse.
Laser radars (ladars) determine range in the atmosphere by measuring the transit time of a laser pulse from the transmitter/receiver to the target and dividing by twice the velocity of light in the atmospheric medium. Range resolution in such devices is related to the accuracy of this transit time measurement. In the atmosphere, ranges are typically measured in kilometers, where range resolution can be as small as 30 cm. A 3D target image can be obtained with a laser radar by rastering the laser beam across the target and measuring the transit time, pulse by pulse, where each pulse corresponds to a point on the target. The distance between points on the target determines the spatial resolution of the rastered image and defines the picture element (pixel) size; the number of pixels at the target determines the pixel-array size; the range resolution determines resolution in the third target dimension. Rastering is a slow process, particularly for large pixel-array sizes, and it requires cumbersome mechanical scanners and complex pixel-registration computer processing.
When high-speed imaging is required, it is desirable to obtain the entire image with one laser pulse. However, because of weight, cost and complexity problems, it is undesirable to obtain the entire image with independent, parallel, laser radar receivers, where each pixel uses a separate laser radar receiver system; multiplexing with one laser radar receiver should be used to make a large pixel-array imaging system practical. Currently there are no lightweight, small-size, multiplexing laser radar receivers which can image an entire target, in the atmosphere, with a single laser pulse at large pixel-array size and high-range resolution. Reference 1, however, discloses a lightweight, multiplexing laser radar receiver which can image an entire target, in water, with a single laser pulse. It accomplishes this feat by storing the reflected signal from each range resolution interval to determine, by means of a signal amplitude comparison, the time of laser pulse returned.
Because the range of lasers in the atmosphere is 10 to 100 times the range in water, but the range resolutions are comparable, it is impractical to store the signal from the too-numerous range resolution intervals and evaluate transit time to a target. Instead the returning pulse must stop a clock. This clock can be a voltage ramp; that is a voltage which is decreasing in time. The voltage at which the ramp is switched off determines the time at which the laser pulse returned. The voltage ramp begins when the laser pulse is transmitted. Because stopping a clock or switching off a voltage ramp is usually dependent upon the returning-pulse amplitude for typical laser pulse widths (due to the clock-stopping circuitry) and the returning-pulse amplitude is dependent, pixel by pixel, on the reflectivity of the target, the range must be corrected for amplitude to obtain high range resolution.